NASA apologists say
there's no physical evidence for a conspiracy theory. There's plenty
of evidence, such as the photo and video anomalies.

This reveals a fundamental flaw in the conspiracist understanding
of the nature of hypothesis and proof.

A hypothesis is a statement whose truthfulness is not known, but
which -- if it were true -- would explain some set of
observations. The proof of that hypothesis would be some other
observation (not the one being explained) which would would be seen
if and only if that particular hypothesis were true, and not,
say, some other hypothesis which also explains the first observations.

If you think this sounds a lot like the scientific method, you're
right. The scientific method uses a carefully chosen experiment to
test which of several hypotheses is the right one. The experiment
tries to see by-products or effects that could only be caused
by the hypothesis the scientists are testing.

But the problem comes when conspiracists want to test a
hypothesis. You can't use the initial observation as proof of your
hypothesis. This is a fallacy -- an example of erroneous thinking --
which logicians call a "circular argument". The notion of a circular
argument can be best summed up in the following fanciful dialogue:

Sir Bedevere: Why are you trying to burn that woman?Villagers: Because she is a witch!Bedevere: How do you know she is a witch?Villagers: Well, we wouldn't be trying to burn her if she
weren't.

Let's say, for example, that I observed my car windshield was wet.
I might hypothesize that it has recently rained. But how would I
prove that? If I were like the villagers in the exchange above, I
would consider it already proved: The wet windshield proves it rained.
But the wet windshield was the observation I was trying to explain.
To know whether or not it rained I would need to look for other
signs of recent rain. For example, I could look at the sky and see if
it's cloudy. Or I could see if the distant surroundings were also
wet. Or I could ask somebody who may have witnessed the rain.

I have to do that because there are lots of other hypotheses.
Perhaps some sort of moisture has leached out of the glass. Perhaps a
nearby sprinkler doused the car. I have to find some way of choosing
one hypothesis over the other. I can't just cite the wet windshield
as evidence. I have to find evidence that doesn't have anything to
do with the windshield itself, but has to do with the process I
hypothesize.

With the circular argument I can put each of these hypotheses into
a syllogism and say,

My windshield is wet, therefore it has rained.

My windshield is wet, therefore moisture has leached out of the
glass.

My windshield is wet, therefore a sprinkler has sprayed my car.

All three of these seem reasonable, but they all can't be correct. I
can be absurd and hypothesize that my windshield is wet because space
aliens controlled by G. Gordon Liddy and Rosie O'Donnell are spying on
me and put that moisture there to absorb my brainwave patterns. The
resulting "proof" would be

Yes, that's supposed to sound absurd. It's supposed to show that a
pattern of reasoning which can produce such absurd conclusions isn't a
valid pattern of reasoning. Circular arguments are "tautological"
meaning that they're always true. They're true not because they
arrive at a good conclusion, they're true because they're structured
to be true no matter what conclusion or premise is involved. That's
why they aren't useful for proving anything.

Let's do an example that involves Apollo data.

Conspiracists observe that in certain Apollo photographs the
"fiducials" or crosshairs seem to pass behind objects in the photo.
They hypothesize that the photos were produced in a laboratory by cut
and paste techniques. This hypothesis, if true, would explain the
observation. If a photo lab technician pasted an object into a photo
already containing fiducials, he might obscure a fiducial by
overlaying his addition on top of it.

But how to go about proving it? Unfortunately most conspiracists
simply use the circular argument. When asked to provide evidence that
a photo lab pasted up the Apollo photos, they point back to the
missing fiducials and say, "See? The fiducials are missing, therefore
they were created in the lab."

But reasonable people are not convinced by the tautological
argument, nor should they be. When we say there is no evidence for
such hypotheses, we mean that there is no evidence which undeniably
and unquestionably shows that a photo lab produced the photographs.
The conspiracists need to provide secondary, unambiguous evidence that
could only be explained by their hypothesis. For example, they could
try to find increased activity at government-funded photo labs. They
could try to document the purchase of equipment and supplies by NASA
that would only be useful to a photo lab that was falsifying pictures
instead of just developing them. They could even try to find some of
the people who participated in this alleged falsification of photos.

The absence of the fiducial doesn't prove the existence of a
previously unknown photo lab any more than the villagers' desire to
burn a woman proves she's a witch.